1. Field of the Invention
The invention relates to a piezoelectric actuator, for instance for actuating a mechanical component such as a valve or the like.
The invention relates to a piezoelectric actuator, for instance for actuating a mechanical component such as a valve or the like, in accordance with the generic characteristics of the preamble to the main claim.
2. Description of the Prior Art
The aforementioned piezoelectric actuators are often used in the positioning of valves. Among other factors, it must be considered here that their stroke capacity for actuating a valve tappet, for instance, is relative slight for a comparatively great force. To increase the useful stroke, it is therefore known to provide a mechanical or hydraulic travel booster. Such mechanical or hydraulic travel boosting systems, however, entail greater effort and therefore greater expense as well.
It is widely known that by utilizing the so-called piezoelectric effect, a piezoelectric element can be constructed from a material with a suitable crystalline structure. When an external electrical voltage is applied, a mechanical reaction of the piezoelectric element takes place, which depending on the crystalline structure and the regions where the electrical voltage is applied causes a compression or tension in a predeterminable direction.
The aforementioned piezoelectric actuators are often used in the positioning of valves. Among other factors, it must be considered here that their stroke capacity for actuating a valve tappet, for instance, is relative slight for a comparatively great force. To increase the useful stroke, it is therefore sometimes usual to provide a mechanical or hydraulic travel booster. Such mechanical or hydraulic travel boosting systems, however, entail greater effort and therefore greater expense as well.
The piezoelectric actuator described at the outset advantageously has at least one piezoelectric element, which is suitable for imposing a tensile force or compressive force on an actuating element. According to the invention, stabilizing elements are provided, which are disposed parallel to the piezoelectric element with a flexible intermediate layer located between the elements, wherein the piezoelectric element and the stabilizing elements have a great length in the effective direction (Z axis) in proportion to their width transversely to the effective direction (X, Y direction). An advantageous order of magnitude would for instance be a ratio of length (Z direction) to width (X, Y direction) is approximately 5:1 to 50:1.
In a first advantageous embodiment, the stabilizing elements are of steel and are held between a base or support plate fastened firmly in the housing of the piezoelectric actuator and a fixation edge in the housing. The housing here is as a rule also made from steel. The piezoelectric element is held between the base plate and a spring plate which, via a prestressing spring, likewise rests on the housing and guides the actuating element.
With the invention, a long, narrow piezoelectric actuator is created in a simple way that is mechanically relatively invulnerable, for instance to vibration when used in the engine compartment of a motor vehicle. Because of the long stroke resulting from the long, narrow design, a stroke boost can be dispensed with, and in principle, tensile forces or compressive forces can be generated by the piezoelectric actuator.
Because a flexible intermediate layer, for instance of plastic such as a polymer or the like, is mounted between the stabilizing elements and the piezoelectric element, a longitudinal motion, which represents a relative motion between the piezoelectric element and the stabilizing element, can be allowed. An oscillating motion of the piezoelectric element in the X or Y direction can be avoided, however. It is thus possible in a simple way to prevent bending tensions in the piezoelectric element that could possibly lead to the destruction of the piezoelectric actuator.
The aforementioned concrete embodiment furnishes downward-oriented compressive forces and is not temperature-compensated. In a second embodiment, the piezoelectric element and the stabilizing elements are of ceramic materials, which have essentially the same coefficients of temperature expansion, so that this embodiment is temperature-compensated. The stabilizing element is here as well held between a base plate and a fixation edge in the housing, but the base plate rests on the housing via a spring.
In this last embodiment, the stabilizing element is mechanically coupled to the piezoelectric element in such a way that the temperature-dictated expansions of the piezoelectric element and of the stabilizing elements cancel one another out in the effective direction in such a way that the actuating element remains in its position, or in other words no zero-point drift occurs. The piezoelectric element is held between the base plate and a spring plate, which via a prestressing spring likewise rests on the housing and guides the actuating element. Here the force of the prestressing spring must be substantially greater than that of the spring on the base plate, so that the different temperature expansions between the housing and the material comprising the piezoelectric element are compensated for via the spring. Additional provisions for temperature compensation, as was often provided until now by hydraulic coupling, are no longer necessary here.
In the invention, the piezoelectric element can be constructed of transversely stacked piezoelectric layers and thus exerts a compressive force on the actuating element, or of longitudinally stacked piezoelectric layers and thus exerts a tensile force on the actuating element.
In other advantageous embodiments, the stabilizing element comprises piezoelectric layers, each located perpendicular to the layered structure of the piezoelectric element, which piezoelectric layers are triggered with a voltage in the same way as the piezoelectric element. With this embodiment, in addition to the temperature compensation, the useful stroke is also lengthened by means of the additional stroke of the stabilizing elements. The contacting of the piezoelectric layers can be located in the intermediate layer or outside it.
In another advantageous embodiment of the invention, two piezoelectric elements are disposed symmetrically to a tension rod, acting as the actuating element, surrounded by the intermediate layer in the housing of the piezoelectric actuator. The piezoelectric elements here are held between a support plate, connected to the tension rod, and a fixation edge in the housing, and the support plate rests on the housing via a spring.
These and other characteristics of preferred refinements of the invention will become apparent from the description and the drawings; the individual characteristics, each alone or a plurality of them in the form of subsidiary combinations, can be realized in the embodiment of the invention and in other fields and can represent both advantageous and intrinsically patentable embodiments for which patent protection is here claimed.